The long term objective of this project is to analyze the mechanisms by which parathyroid hormone (PTH) controls bone remodeling and stimulates bone resorption. The discovery that PTH does not directly stimulate the osteoclast, but rather increases resorptive activity through its actions in other cells (most likely osteoblasts), has left the mechanisms of PTH action unresolved. Since PTH is one of the most important systemic regulators of bone resorption, knowledge of its mechanism of action is critical to understanding disease states of altered bone resorption such as osteoporosis. In addition, the means by which PTH augments bone formation through actions in the same lineage of cells it uses to encode messages augmenting resorption is unclear. Recent work of the principal investigator, supported through the initial funding cycle of this application has demonstrated a bi-functional mechanism of signal transduction activated by PTH. The PTH-receptor complex couples to adenylate cyclase increasing cAMP production, but it also activates a phospholipase-C leading to production of inositol phosphates, diacylglycerol, and transient elevations of cytosolic calcium. The specific aims of the renewal application are to analyze the phospholipase-C activated by PTH in osteoblasts, and determine the role of this message system in osteoblast function. Studies examining activation of phospholipase C at specific stages in the cell cycle and at different stages of osteoblast differentiation may provide critical information regarding the anabolic potential of PTH. To examine whether the anabolic effects of PTH on bone formation are transmitted by augmentation of load induced signals, the interaction between physical strain induced osteoblast function and PTH will be analyzed. The studies in the proposal involve analysis of [Ca2+]i stimulated by PTH using digital imaging microscopy, patch-clamp recordings of single ion channels in osteoblast, analysis of the effects of PTH on cell-cell contact and communication, and characterization of the components of phospholipase-C activated by PTH. The latter studies will include the use of molecular biologic approaches to determine the nature of the phospholipase-C activated by PTH and characterize the GTP binding proteins associated with the PTH receptor-phospholipase-C complex. The studies proposed in this application, will improve our understanding of osteoblast function regulated by PTH through phospholipase-C. The studies will contribute significantly to our understanding of disease states characterized by alterations in PTH function, i.e. osteoporosis and renal osteodystrophy.